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Components and Cladding Vs. Main Windforce Resisting System

CJJS (Structural) (OP)
23 Mar 07 10:03
There has been some discussion in other forums on C&C vs. MWFRS.  I am curious as to how different truss manufacturers are analyzing roof trusses.  Here, we analyze them using the MWFRS criteria.  After reading the other forums I am starting to believe that they may need to be analyzed using C&C.  
KeySol (Structural)
23 Mar 07 14:07
It is my understanding that you need to design truss chords and webs with the C&C loading. The roof truss itself is a component of the building. The MWFRS of the building are the shearwalls.
woodengineer (Structural)
23 Mar 07 21:58
KeySol is correct.  In MiTek, for example, under wind design we choose a hybrid method which analyzes the components chords and webs as C&C and the reactions (uplifts) as MWFRS.

woodengineer

Trussme68 (Structural)
31 Mar 07 10:35
Truswal analyzes both CC and MWF. It is up to the building designer to tell us which to report on the drawings. We can report CC, MWF, or both on the truss drawings. Sometimes it is easier to just report one to eliminate confusion by building inspectors, but trusses are still run with both cases in the background.
MonteSanoKid (Structural)
21 Jul 07 10:15
With Keymark's KeyTruss software, we analyze for both all the time, and report both all the time.  As might be expected, this does generate some confusion.

I also agree that the holdowns should be desgined only for MWFRS, but I find many building designers who feel differently about this.
UcfSE (Structural)
21 Jul 07 12:10
ASCE gives clear examples that typical trusses are intended to be C&C.  If they are part of the MWFRS (not the normal case), as in the case of a drag strut, then you have an additional check to make with MW forces.  If the effective tributary area is greater than 700 ft2, then it can be ran as MW.  
JAE (Structural)
21 Jul 07 13:25
I agree with UcfSE.  Much of the decision rests upon how much tributary area the particular element you are designing takes.

CFSEng (Structural)
23 Jul 07 9:35
I dont agree that truss support reactions should be designed for C&C loading.  C&C loading occurs on one surface when the wind comes from the worst direction possible for that surface.  So it is not possible to have zone 2/3 pressures occuring on both ends of a truss and both sides of a ridge simultaneously.  Which version of ASCE provides the clear examples you mention?
JAE (Structural)
23 Jul 07 9:39
CFSEng,

I'm not sure I follow your reasoning.  If you have a truss that spans from wall to wall, and the wind blows from one direction, then the truss support reaction on that side feels that load and sometimes it's a zone 2 or 3 wind - an extreme load due to limited tributary area on that connection.

Later, the wind may blow from the other direction and the other support connection would then feel a high localized wind pressure.  

No, they don't occur simultaneously but they do occur separately and each connection should be designed for the worst case.  

Can you clarify?  Thanks.

csd72 (Structural)
23 Jul 07 10:22
UcfSE (Structural)
23 Jul 07 11:45
CFS, check the ASCE 7 commentary and the Guide to the Use of the Wind Load Provisions of ASCE 7-02.

The rest of your post isn't very clear, or doesn't seem to make sense.  Would you mind clarifying?
CFSEng (Structural)
23 Jul 07 13:27
The paper in csd72 post does a much better job stating what I was trying to say

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